1. Field of the Invention
The present invention relates to an ink jet apparatus adapted to achieve a printing operation in a multi-gray scale mode by performing scanning operations by several times with the aid of an ink jet head having a plurality of ink ejection orifices formed thereon. Further, the present invention relates to an ink jet method to be practiced by operating an ink jet apparatus of the foregoing type.
It should be noted that the ink jet apparatus and the ink jet method to which the present invention has been applied should not be limited only to an ink jet apparatus and an ink jet method both of which are employable for recording characters, images or the like on a paper. Alternatively, the present invention may equally be applied to an ink jet apparatus and an ink jet method both of which are employable for ejecting ink to a cloth or a various medium and fixing thereto (hereinafter referred to widely as printing).
2. Description of the Prior Art
There have been hitherto known various kinds of methods each capable of expressing a gray scale of each image or the like in accordance with an ink jet system.
The conventional methods as mentioned above are exemplified by a method of changing an area of each pixel by changing a size of each ink droplet ejected from an ink ejection orifice (Area gray scale method), a method of expressing a gray scale by changing the number of dots on the assumption that an assembly of dots formed with ejected ink droplets is counted as one pixel (Density pattern method, Dither method or the like), a method of performing a recording operation using inks each having a different density of color (Different density method), and a method of expressing a gray scale by changing an area of each dot or a density of color of the dot depending on the number of ink droplets shot onto a recording medium on the assumption that one dot is formed by shooting a plurality of ink droplets onto the recording medium (multi-droplet method). A term "shot" means that ink for forming a pixel is ejected to the pixel. Among the aforementioned conventional methods, the multi-droplet method is expected as a method of enabling each recording operation or each printing operation to be performed at a high speed while expressing a high resolution and a great many gray levels.
However, since the conventional multi-droplet system is practiced such that a single pixel is formed with ink droplets ejected from a same ejection orifice, in the case that the direction of ejecting a series of ink droplets and a quantity of each ejected ink fluctuate (variation) from ejection orifice to ejection orifice, there arise problems that stripes (bandings) appear on an image to be originally uniform in structure, and moreover, a shade of color varies. Specifically, when the ink droplet flying direction fluctuates, the position of each dot formed on a recording paper is dislocated from a predetermined one, resulting in stripes undesirably appearing on a recorded image. In addition, when a volume of each ejected ink droplet fluctuates, a size of each dot formed on the recording paper and density of color of the same fluctuate, resulting in the shade of color of each image formed on the recording paper undesirably fluctuating.
To prevent the aforementioned problems from arising, the conventional multi-drop system requires that an ink jet head is produced at a very high accuracy in order to suppress fluctuation of the ink droplet ejecting direction and fluctuation of an ink droplet ejecting volume between adjacent ejection orifices as far as possible. However, this leads to another problems that the ink jet head is produced at an expensive cost and a yielding rate associated with the production of the ink jet head is degraded.
There is exemplified a method of solving a problem of fluctuation of a shade of color by employing a suitable software. This method is achieved by changing the number of ink droplets to be shot onto a recording medium in such a manner as to eliminate a difference between adjacent ejection orifices in a quantity of ink to be ejected therefrom. However, when the exemplified method is practically employed for an ink jet recording system, there arises a problem that the whole system is produced at an increased cost. In addition, the employment of the foregoing method is not effective for solving a problem of appearance of stripes or bands on a recorded image. In the case that fluctuation between adjacent ejection orifices in a quantity of ink to be ejected therefrom varies as time elapses, it is necessary to readjust the number of ink droplets to be shot onto the recording medium. This leads to another problem that a quality of maintenance service to be rendered for the ink jet apparatus is undesirably degraded.
To solve the foregoing problem, the assignee to the present invention already proposed a multi-gray level recording method such that a pixel is formed with a plurality of ink droplets which are ejected from different orifices from each other so that fluctuation between pixels in a quantity of ejected ink can be reduced and moreover, bands and shade irregularity visually recognizable on a recorded image is appeared with much difficulties as disclosed in Japanese Patent Application Laying-open No. 4-358847 (U.S. Ser. No. 07/893,086).
FIG. 1 is an illustrative view which schematically shows an outline of the proposed method.
Specifically, when this method is practiced for an ink jet apparatus, an image is recorded on a recording medium by performing scanning operations by several times, and at the same time, respective dots constituting a line extending in the scanning direction are formed with a plurality of ink droplets ejected from different ejection orifices during the several scanning operations. For example, in the case that a plurality of black-colored pixels each represented by a mark of .circle-solid. are recorded on the recording medium, dots each constituting a pixel are formed with ink droplets are ejected from three different ejection orifices from each other, fluctuation of the respective ink droplets in the ejecting direction can be averaged so that any visual recognition of stripes do not appear on a recorded image. In the case that it is assumed that fluctuation of a quantity of each ejected ink droplet among ejection orifices is normally distributed with a standard deviation .sigma., when the proposed method is employed for the recording apparatus, fluctuation of a quantity of ejected ink between adjacent lines is reduced to a level of .sigma./.sqroot.3. Since the fluctuation of a quantity of each ejected ink droplet between adjacent lines is visually recognized as fluctuation of a pixel density, an image having less shade irregularity can be obtained with the recording apparatus.
However, for example, with respect to the case exemplified in FIG. 1, there arises a problem that a recording speed of the recording apparatus is reduced because the number of scanning operations performed for the purpose of recording is tripled.
To cope with this problem, there is proposed a method of reducing the number of scanning operations to be performed for the same purpose and ejecting a plurality of ink droplets from a single ejection orifice per one scanning operation.
FIG. 2 is an illustrative view which show an example of the foregoing proposed method. FIG. 2 shows the case that one pixel is recorded on the recording medium during two scanning operations.
Specifically, in the case that three ink droplets are required per one pixel, two ink droplets during one scanning operation are combined with one ink droplet during one scanning operation, in the case that two ink droplets are required per one pixel, one ink droplet is ejected from one ejection orifice during one scanning operation and scanning operations are performed by two times, and in the case that one ink droplet is required per one pixel, a recording operation is achieved during either of two scanning operations.
When this method is employed for the recording apparatus, scanning operations are performed by two times per one pixel. Thus, this method has an advantage that a recording speed can be increased compared with the aforementioned conventional method. However, in contrast with this conventional method wherein scanning operations are performed by three times to form one pixel, the foregoing method has problems that an extent of eliminating the malfunctions of appearance of stripes and shade irregularity visually recognizable on a recorded image is appreciably insufficient, and moreover, when three ink droplets are shot onto the recording medium, ink is liable to overflow on the recording medium due to a large quantity of ink to be shot onto the latter per a unit time.
With respect to one of the aforementioned problems, i.e., appearance of stripes and shade irregularity visually recognizable on the recorded image, on the assumption that the standard deviation of fluctuation of a quantity of ink ejected from the ejection orifices is designated by .sigma., the fluctuation of a quantity of ejected ink from line to line is represented by .sigma./.sqroot.2 (in the case that one pixel is formed during one scanning operation). Since this fluctuation is not largely different from .sigma./.sqroot.3 (in the case that one pixel is formed during three scanning operations), there are many cases that the foregoing fluctuation is allowable.
On the other hand, with respect to the other problem that ink overflow is liable to occur on the recording medium, a malfunction of comparatively significant image deterioration (hereinafter referred to as bleeding), i.e., a malfunction of formation of an indistinct recorded image of which corners are vaguely recognized is liable to arise especially in the case that each recording operation is performed using a recording medium having poor ink absorption properties or in the case that two kinds of inks each having a different color are mixed with each other to form an indistinct image.